Several lines of evidence point toward circadian involvement in the pathogenesis of human disease. The human circadian system consists of a central pacemaker (suprachiasmatic nucleus) and also peripheral oscillators that are located in nearly every human tissue, including the heart, vessel walls and beta cells of the pancreas. Melatonin is a hormone secreted by the pineal gland (under the control of the suprachiasmatic nucleus) and entrained to the dark phase of a light-dark cycle. Besides serving as an antioxidant, melatonin suppresses sympathetic outflow, promotes vascular relaxation, and affects insulin secretion. As a molecular mediator of the circadian clock, melatonin is inversely associated with the development of hypertension in prospective data, and inversely associated with the prevalence of cardiovascular disease (CVD) in cross- sectional studies; furthermore, polymorphisms in the melatonin receptors are associated with type 2 diabetes mellitus (T2DM). Whether melatonin production is independently associated with future CVD events and incident T2DM, however, has not been studied. Although human genetic studies suggest that common variants in core genes that control the circadian system (e.g., clock, arntl) may influence the development of T2DM and hypertension, associations with CVD have not been reported, and potential interactions between circadian disruption (short sleep duration, rotating night shift work, snoring) and these variants have not been explored. Finally, the relation between elements of the circadian system (i.e., melatonin, circadian genes) and CVD/T2DM mediators in humans is not well understood. The 3 major hypotheses that we will investigate are: (1) lower melatonin production is associated with CVD (myocardial infarction [MI] and stroke) and T2DM; (2) common variants in circadian genes are associated with CVD and T2DM, and the association is modified by circadian-disruptive factors; and (3) both lower melatonin production and circadian gene variants are associated with inflammation, insulin resistance, and endothelial dysfunction. In this study, we propose to draw on both new data collection and extensive existing genetic data from the ongoing Nurses' Health Study (NHS) cohort. Aim 1 will employ two nested case-control studies (400 case-control pairs each) to examine the prospective association between melatonin production and the risk of incident CVD and T2DM events. Aim 2 will employ a large sub-cohort (N=11,587) to investigate whether common genetic variants in circadian control genes are associated with prevalent CVD and T2DM, and prospectively, whether these variants interact with short sleep duration, rotating night shift work, and snoring in the development of CVD and T2DM. Aim 3 will examine whether elements of the circadian system (melatonin, genetic variants) are associated with inflammation, insulin resistance, and endothelial dysfunction. The results from this highly efficient and cost- effective study will help elucidate the relation between circadian misalignment and CVD and T2DM, and ultimately, could help identify novel treatment and prevention strategies.